The various warning methods have been proposed which give a warning when a seat belt is not fastened (for example, see Patent Literature 1 or Patent Literature 2). FIG. 3 is a system block diagram illustrating an example of the warning method according to the related art. As shown in FIG. 3, a warning apparatus according to the related art includes an ignition key switch 10 that controls the driving and stopping of an engine, a seat sensor 20 that detects whether a passenger is sitting on the seat, seat belt switches 30 that detect whether the seat belts are fastened as on/off states, a CPU (that is, a Central Processing Unit) 40, LEDs (that is, Light Emitting Diodes) 50 that give a seat belt warning generated on the basis of a control command from the CPU 40, and a buzzer 60 that makes a warning sound.
The seat belt warning apparatus having the above-mentioned structure according to the related art controls the output of a warning for the driver's seat by a combination of the start-up (IGN+10) of the engine and the seat belt switches 30. That is, when the engine has started up and the seat belt switch 30 indicates that the seat belt is not fastened, the seat belt warning apparatus gives a warning. Meanwhile, the seat belt warning apparatus gives a warning for the front passenger's seat by a combination of the seat sensor 20 and the seat belt switches 30, and gives a warning for the rear seats using the seat belt switches 30. The CPU 40 periodically detects the state of the seat belt switch 30 and stores the state in a seat belt flag (register) 100 of the CPU 40. Therefore, when any one of the seat belts of the rear seats is changed from a fastened state to an unfastened state, an LED corresponding to the rear seat is turned on to display the unfastened state of the seat belt and simultaneously, the buzzer 60 sounds to notify the unfastened state, as shown in FIG. 4.